Method for determining the frequency response of an electrooptical component

ABSTRACT

The aim of the invention is to provide a method for determining the frequency response of an electrooptical component, particularly, for example, of a light-generating or light-modulating component, which is easy to carry out. To this end, the invention provides a method during which optical pulses with a pulse frequency (fp) are generated. The electrooptical component ( 60 ) is controlled by an electrical measuring signal (Smess) with a measuring frequency (fmess) in such a manner that an optical output signal (Saus) is formed that is modulated with the measuring frequency (fmess). The measuring frequency (fmess) is equal to an integral multiple of the pulse frequency (fp) plus a predetermined frequency offset ($g(D)f). The pulses and the output signal (Saus) are mixed, and a mixed product (M) is detected whose modulation frequency corresponds to the predetermined frequency offset ($g(D)f). The mixed product indicates the frequency response of the electrooptical component ( 60 ) at the measuring frequency (fmess).

The invention is based on the object of specifying a method fordetermining the frequency response of an electrooptical component, inparticular for example of a light-generating or light-modulatingcomponent, which can be carried out in a very simple manner.

This object is achieved according to the invention by means of a methodhaving the features in accordance with patent claim 1. Advantageousrefinements of the method according to the invention are specified insubclaims.

Accordingly, the invention provides a method in which optical pulseshaving a first optical carrier frequency and having a predefined pulsefrequency are generated. The electrooptical component whose frequencyresponse is to be determined is driven with an electrical measurementsignal having a predefined measurement frequency in such a way that itforms an optical output signal—modulated with the measurementfrequency—having a predefined second optical carrier frequency. In thiscase, the measurement frequency is chosen in such a way that it amountsto an integral multiple of the pulse frequency of the optical pulsesplus a predefined frequency offset. The optical pulses and the opticaloutput signal are jointly subjected to a frequency mixing, in whichcase, from the optical mixed products formed during the frequencymixing, at least one mixed product is detected whose modulationfrequency corresponds to the predefined frequency offset. The frequencybehavior of the electrooptical component is subsequently determined onthe basis of the quantity, in particular the power, the amplitude or theroot-mean-square value, of the selected mixed product. The detection ofthe mixed product and the determination of the frequency behavior of theelectrooptical component are carried out successively for allmeasurement frequencies which correspond to an integral multiple of thepulse frequency of the optical pulses plus the predefined frequencyoffset and which lie within a predefined frequency band within which thefrequency behavior of the electrooptical component is intended to bedetermined.

One essential advantage of the method according to the invention is thatit can be carried out in a very simple manner since, by way of example,a pulsed laser used for generating the optical pulses only ever has tobe driven with one and the same pulse frequency. Since pulses aregenerated by the pulsed laser, the frequency is spectrum of the opticaloutput signal generated by the pulsed laser has a very wide frequencyspectrum extending into the range of up to hundreds of GHz. In thiscase, the frequency spectrum of the pulsed laser comprises a frequencycomb with a line spacing that corresponds to the pulse frequency. Inother words, the power spectrum of the laser pulses comprises lineshaving frequencies n*fp (fp: pulse frequency), where n denotes aninteger. In this case, each of the spectral lines of the frequency combhas an intensity Rn. The frequency spectrum of the electroopticalcomponent can then be determined for all measurement frequencies whichcorrespond to an integral multiple of the pulse frequency plus apredefined frequency offset (e.g. 1 kHz). When the optical signalsgenerated by the pulsed laser and the electrooptical component aremixed, a signal having a modulation frequency which corresponds to thepredefined frequency offset occurs, inter alia. By measuring at leastone mixed product whose modulation frequency corresponds to thefrequency offset, the frequency behavior of the electrooptical componentcan thus be ascertained for each measurement frequency.

To summarize, the method according to the invention thus has theadvantage that the frequency response of the electrooptical componentcan be determined for different measurement frequencies even though itis only ever necessary to evaluate one measurement quantity having oneand the same modulation frequency, namely having the predefinedfrequency offset.

The predefined frequency offset which defines the mixed products to bedetected may have a positive or negative magnitude. This means that afrequency which may amount to an integral multiple of the pulsefrequency of the optical pulses plus or minus a predefined (positive)frequency offset may be chosen as the measurement frequency.

Preferably from the mixed products, exclusively those mixed products aredetected which have the summation frequency formed from the first andsecond optical carrier frequencies as optical carrier frequency.

As an alternative, but likewise preferably, from the mixed products,exclusively those mixed products are detected which have the differencefrequency formed from the first and second optical carrier frequenciesas optical carrier frequency.

In order to achieve a particularly high measurement accuracy, it isregarded as advantageous if the spectral line strengths of the opticalpulses are determined beforehand and if they are taken into account whendetermining the frequency behavior of the electrooptical component. The“spectral line strengths” may be determined e.g. by Fouriertransformation of the autocorrelation of the optical pulses.

When determining the frequency behavior of the electrooptical component,from the spectral line strengths of the optical pulses that have beendetermined beforehand, the spectral line strength of in each case thatspectral line whose spectral line frequency corresponds to thedifference frequency between the respective measurement frequency andthe predefined frequency offset is taken into account.

The spectral line strengths of the optical pulses can be taken intoaccount in a particularly simple manner and hence advantageously bymeans of a mixed product intensity value that specifies the intensity ofthe selected mixed product being divided by a spectral line valuespecifying the spectral line strength of the spectral line of theoptical pulses which is associated with the selected mixed product. Afrequency response value of the electrooptical component is in each caseformed by this division.

A nonlinear element through which the optical pulses and the opticaloutput signal are radiated is used for the purpose of forming theoptical mixed products.

As an alternative, by way of example, a 2-photon detector may also beused for the purpose of forming and/or detecting the optical mixedproducts.

Moreover, an optical rectifier, in particular for example a nonlinearcrystal, may also be used for the purpose of forming and/or detectingthe optical mixed products.

The measurement frequency may preferably be calculated in accordancewith the following determination equation:fmeas=m*fp+Δfwhere fmeas denotes the measurement frequency, Δf denotes the frequencyoffset and fp denotes the pulse frequency.

The method according to the invention can be used for example todetermine the frequency response of an electrooptical component formedfrom a light source, in particular a laser (e.g. an unpulsed CW laser)or a light-emitting diode, and a modulator. The modulator may be forexample a drivable modulator, that is to say for example anelectrooptical, electroacoustic or similar modulator. If an unpulsedlaser is used as the light source, then primarily the frequency responseof the modulator is measured when carrying out the method according tothe invention.

Moreover, in an advantageous manner, the frequency response of anoptoelectrical transducer is simultaneously determined by radiating theoptical output signal generated by the electrooptical component into theoptoelectrical transducer, measuring an electrical transducer signalgenerated by the optoelectrical transducer with formation of atransducer measured value, and using the transducer measured value andthe measured frequency response of the electrooptical component todetermine the frequency response of the optoelectrical transducer.

In this case, the frequency response of the optoelectrical transducermay be derived in a particularly simple manner and hence advantageouslyby means of the transducer measured value being divided by a frequencyresponse value of the electrooptical component.

Preferably, the pulse frequency of the optical pulses is generated bymeans of a pulse generator and the measurement frequency of themeasurement signal is generated by means of a sine-wave generator, thetwo generators being synchronized, for example being coupled inphase-locked fashion.

Moreover, in accordance with one advantageous development of the method,the phase response of the electrooptical component may additionally bemeasured.

For this purpose, preferably, a phase signal is generated whichspecifies the phase difference between the drive signal of the pulsedlaser and the electrical measurement signal. The phase angle between thegenerated phase signal and the detected mixed product is subsequentlymeasured for each of the measurement frequencies in each case withformation of a phase measured value.

The phase response of the optoelectrical transducer may also be measuredin a corresponding manner.

The invention is furthermore based on the object of specifying anarrangement that can be used to determine the frequency response of an,in particular, light-generating or light-modulating electroopticalcomponent in a very simple manner.

This object is achieved according to the invention by means of anarrangement having the features in accordance with patent claim 20.

With regard to the advantages of the arrangement according to theinvention, reference is made to the above explanations in connectionwith the method according to the invention.

For elucidating the invention:

FIG. 1 shows a first exemplary embodiment of an arrangement according tothe invention that can be used to carry out the method according to theinvention,

FIG. 2 shows a second exemplary embodiment of an arrangement accordingto the invention, in which the phase response of an electroopticalcomponent can additionally be determined, and

FIG. 3 shows a third exemplary embodiment of an arrangement according tothe invention.

FIG. 1 reveals an electrical high-frequency source 10 (e.g. pulsegenerator), which drives a pulsed laser 20.

The pulsed laser 20 is connected by an optical waveguide 30 to anonlinear crystal 40 to which a photodetector 50 is coupled on theoutput side. The nonlinear crystal 40 is connected by means of a furtheroptical waveguide 55 to an electrooptical component 60, which may be forexample a light-emitting diode or a laser.

The electrical driving of the electrooptical component 60 is effected bya second electrical high-frequency source 70 (e.g. sine-wave generator),which is connected to the first electrical high-frequency source 10 bymeans of a synchronization line 80. A synchronization signal FT istransmitted via the synchronization line 80. The synchronization signalFT may have a frequency of 10 MHz, by way of example.

The arrangement in accordance with FIG. 1 is operated as follows:

The laser 20, which may be for example a short-pulse laser with lowphase noise, is driven by the first electrical high-frequency source 10with a drive signal SA in such a way that the laser 20 generates shortlaser pulses with a repetition rate fp. The power spectrum of theseoptical laser pulses thus comprises a frequency comb with a line spacingfa where fa=fp, that is to say thus comprises spectral lines havingfrequencies n*fp, where n denotes an integer. The spectral lines havingthe frequencies n*fp in each case have the intensity In.

In this case, the full width at half maximum of the laser pulses ischosen such that a sufficiently strong spectral line still remains orexists at the maximum required measurement frequency for thecharacterization of the electrooptical component 60 within a predefinedfrequency band. This is possible without any problems, however, up tofrequencies of hundreds of GHz, since correspondingly short pulses canreadily be generated by commercially available pulsed lasers.

The exact strength or intensity of the individual spectral lines of theline spectrum of the pulsed laser 20 can be measured without anyproblems and with high accuracy up to frequencies in the terahertz rangewith the aid of a so-called autocorrelator, which is likewisecommercially available. In this case, the spectral line strengths areformed by the Fourier transform of the autocorrelation of the opticalpulses.

The frequency response of the electrooptical component 60 is thendetermined as follows: the electrooptical component 60 is successivelydriven in each case with a measurement signal Smeas having the frequencyfmeasfmeas=m*fp+Δf(m=1, 2, . . . ; Δf=const.)where Δf denotes a predefined, constant frequency offset.

The electrooptical component 60 then generates, at the respectivefrequency fmeas, an optical output signal Sout having the intensity Dm,where the quantity Dm describes the frequency behavior to be determinedof the electrooptical component 60 at the measurement frequency fmeas.

The optical pulses of the pulsed laser 20 and also the optical outputsignal Sout of the electrooptical component 60 are then radiated intothe nonlinear crystal 40 via the optical waveguides 30 and 55, with theresult that a mixing or frequency mixing of the signals occurs. A mixedsignal M is then formed which has the following modulation Mod:${Mod} = {\sum\limits_{n}{I_{n}{D_{m}\left( {{\left\lbrack {n - m} \right\rbrack f_{p}} + {\Delta\quad f}} \right)}}}$

The generated mixed signal M is measured by means of the photodetector50 with formation of a photodetector signal M′. An HF measuring device100 having a filter 110 and an evaluation device 120 is connected to thephotodetector 50 on the output side. The filter 110 only transmits thefrequency Δf, that is to say the frequency corresponding to thefrequency offset. The remaining frequencies, for example the frequencyfp or multiples of this frequency, are not transmitted. Thus, all thatremains from the modulation “Mod” is the portion for n=m, so that theevaluation device 120 of the HF measuring device 100 detects or utilizesonly the mixed product M″ having the predefined frequency offset Δf asmodulation frequency.

The mixed product M″, which has the predefined frequency offset Δf asfrequency and whose magnitude is proportional to the intensity Im*Dm, isthus obtained at the output of the filter 110 of the HF measuring device100. Since—as explained above, the spectral line strengths of the pulsedlaser 20 and thus the factor Im have already been determined by means ofthe autocorrelation measurement, the quantity Dm apart from theproportionality factor A can be determined directly from the filteredmixed product M″ in accordance withDm*A=(A*Im*Dm)/Im

If this measurement is then carried out for all values of m for whichthe measurement frequency fmeas lies within a predefined frequency band,then the complete frequency response of the electrooptical component 60is obtained for this predefined frequency band.

A wide variety of components such as, for example, laser diodes,light-emitting diodes and laser-modulator units may be characterized aselectrooptical components 60.

FIG. 2 shows a modification of the arrangement in accordance withFIG. 1. In addition to the components already explained in connectionwith FIG. 1, the illustration reveals a first phase angle measuringdevice 200, which, on the input side, is connected to the output of thehigh-frequency source 10 and to the output of the second high-frequencysource 70. On the output side, the first phase angle measuring device200 is connected to an input E210 a of a second phase angle measuringdevice 210, the other input E210 b of which is connected to the outputof the filter 110.

The phase response of the electrooptical component 60 is additionallymeasured by means of the second phase angle measuring device 210. Forthis purpose, the first phase angle measuring device 200 generates aphase signal PL1 specifying the phase angle ΔΦ1 between the drive signalSA of the pulsed laser 20 and the electrical measurement signal Smeas.

The second phase angle measuring device 210 measures the phase angle ΔΦ2between the generated phase signal PL1 and the phase angle ΔΦm of thefiltered-out mixed product m″ for each of the measurement frequenciesfmeas in each case with formation of a phase measured value ΔΦtot(fmeas). The phase measured values ΔΦtot(fmeas) specify the phase response ofthe electrooptical component 60.

The phase measured values ΔΦtot are transmitted from the second phaseangle measuring device 210 to the evaluation device 120, where they areevaluated and processed further.

The arrangements in accordance with FIGS. 1 and 2 may for example alsobe used to characterize an electrooptical component 60 formed by a lightsource, e.g. a CW laser, and a modulator. Since the CW laser willgenerally be less frequency-dependent than the modulator, the mixedproduct M″ at the output of the filter 110 will essentially describeonly the frequency response of the modulator.

FIG. 3 reveals a further modification of the arrangement in accordancewith FIG. 1 as a third exemplary embodiment. It can be discerned in FIG.3 that the electrooptical component 60 to be characterized is formed bya light source 61, e.g. a CW laser, and a modulator 62.

The modulator 62 of the electrooptical component 60 is connected via athird optical waveguide 300 to an optoelectrical transducer 400, whichmay be a photodetector, by way of example. The optical output signalSout generated by the electrooptical component 60 thus passes via thethird optical waveguide 300 additionally to the optoelectricaltransducer 400, which measures the output signal Sout with formation ofa measurement signal or transducer signal M2 and transmits themeasurement signal M2 to the HF measuring system 120.

The HF measuring system 120 then measures, by means of the photodetector50, firstly the frequency behavior of the electrooptical component 60.Afterward, the electrical measurement signal M2 of the optoelectricaltransducer 400 is then evaluated in the HF measuring system 120, so thatthe frequency response of the optoelectrical transducer 400 is alsodetected metrologically. In this case, the frequency behavior or thefrequency response of the electrooptical component 60 is taken intoaccount since the measurement signal M2 represents a type of“superposition” of the frequency response of the electroopticalcomponent 60 and the frequency response of the optoelectrical transducer400. By virtue of the fact that firstly the frequency behavior of theelectrooptical component 60 is determined, this can be “calculated out”from the measurement signal M2 by the HF measuring system 120, so thatsolely the frequency response of the optoelectrical transducer 400 canbe determined despite the “superposition”.

The frequency response of the electrooptical component 60 isdetermined—as explained above—by means of the photodetector 50 and thefilter 110. Since the CW laser 61 will generally be lessfrequency-dependent than the modulator 62, the mixed product M″ at theoutput of the filter 110 will essentially describe the frequencyresponse of the modulator 62.

Moreover, the phase response of the optoelectrical transducer 400 canalso be measured by using at least one additional phase angle measuringdevice which measures the phase angle between the mixed product M″ andthe electrical measurement signal M2 of the optoelectrical transducer400 or else between the phase angle PL1—as explained in connection withFIG. 2—and the electrical measurement signal M2 of the optoelectricaltransducer 400 and transmits the respective measurement signal to theevaluation device 120. The “additional” phase angle measuring device isnot illustrated in FIG. 3 for the sake of clarity.

LIST OF REFERENCE SYMBOLS

-   10 First high-frequency source-   20 Pulsed laser-   30 First optical waveguide-   40 Nonlinear crystal-   50 Photodetector-   55 Second optical waveguide-   60 Electrooptical component-   61 CW laser-   62 Modulator-   70 High-frequency source-   80 Synchronization line-   100 HF measuring system-   110 Filter-   120 Evaluation device-   300 Third optical waveguide-   400 Optoelectrical transducer.

1. A method for determining the frequency response of an electroopticalcomponent (60) within a predefined frequency band, in which opticalpulses having a first optical carrier frequency and having a predefinedpulse frequency (fp) are generated, the electrooptical component (60) isdriven with an electrical measurement signal (Smeas) having a predefinedmeasurement frequency (fmeas) in such a way that an optical outputsignal (Sout)—modulated with the measurement frequency (fmeas)—having apredefined second optical carrier frequency is formed, the measurementfrequency (fmeas) being an integral multiple of the pulse frequency (fp)plus a predefined frequency offset (Δf), the pulses and the outputsignal (Sout) are subjected to a joint frequency mixing and, from themixed products formed during the frequency mixing, at least one mixedproduct (M″) is detected whose modulation frequency corresponds to thepredefined frequency offset (Δf), the frequency behavior of theelectrooptical component (60) at the measurement frequency (fmeas) isdetermined on the basis of the intensity, in particular the power, theamplitude or the root-mean-square value, of the detected mixed product(M″), and the frequency behavior of the electrooptical component (60) isdetermined in the manner described for all measurement frequencies(fmeas) which correspond to an integral multiple of the pulse frequency(fp) plus the predefined frequency offset (Δf) and which lie within thepredefined frequency band.
 2. The method as claimed in claim 1,characterized in that, from the mixed products, exclusively those mixedproducts (M″) are detected which have the summation frequency formedfrom the first and second optical carrier frequencies as optical carrierfrequency.
 3. The method as claimed in claim 1, characterized in that,from the mixed products, exclusively those mixed products are detectedwhich have the difference frequency formed from the first and secondoptical carrier frequencies as optical carrier frequency.
 4. The methodas claimed in claim 1, characterized in that the predefined frequencyoffset (Δf) has a positive or a negative magnitude.
 5. The method asclaimed in claim 1, characterized in that the spectral line strengths ofthe optical pulses are determined beforehand and they are taken intoaccount when determining the frequency behavior of the electroopticalcomponent (60).
 6. The method as claimed in claim 5, characterized inthat when determining the frequency behavior of the electroopticalcomponent (60), from the spectral line strengths of the optical pulsesthat have been determined beforehand, the spectral line strength of ineach case that spectral line whose spectral line frequency correspondsto the difference frequency between the respective measurement frequency(fmeas) and the predefined frequency offset (Δf) is taken into account.7. The method as claimed in claim 1, characterized in that the spectralline strengths determined beforehand are determined by means of thespectral power of the spectral lines of the optical pulses beingdetermined beforehand, in particular by means of an autocorrelator. 8.The method as claimed in claim 6, characterized in that, for the purposeof determining the frequency behavior of the electrooptical component(60), a mixed product intensity value (Im*Dm) specifying the intensityof the selected mixed product (M″) is divided by a spectral line value(Im)—specifying the spectral line strength of the spectral line of theoptical pulses which is associated with the selected mixed product(M″)—with formation of a frequency response value (Dm) of theelectrooptical component (60).
 9. The method as claimed in claim 1,characterized in that a nonlinear element (40) through which the opticalpulses and the optical output signal (Sout) are radiated is used for thepurpose of forming the optical mixed products (M).
 10. The method asclaimed in claim 1, characterized in that a 2-photon detector is usedfor the purpose of forming and/or detecting the optical mixed products.11. The method as claimed in claim 1, characterized in that an opticalrectifier, in particular a nonlinear crystal, is used for the purpose offorming and/or detecting the optical mixed products.
 12. The method asclaimed in claim 1, characterized in that the measurement frequency iscalculated in accordance with the following determination equation:fmeas=m*fp+Δf where fmeas denotes the measurement frequency, Δf denotesthe frequency offset and fp denotes the pulse frequency.
 13. The methodas claimed in claim 1, characterized in that the predefined frequencyoffset (Δf) is predefined in a variable fashion.
 14. The method asclaimed in claim 1, characterized in that the frequency response of anelectrooptical component (60) formed from a light source (61) and adownstream electrooptical modulator (62) is determined.
 15. The methodas claimed in claim 1, characterized in that the frequency response ofan optoelectrical transducer (400) is simultaneously determined withinthe predefined frequency band by radiating the optical output signal(Sout) generated by the electrooptical component (60) into theoptoelectrical transducer (400), measuring an electrical transducersignal (S2) generated by the optoelectrical transducer (400) withformation of a transducer measured value, and using the transducermeasured value and the measured frequency response of the electroopticalcomponent (60) to determine the frequency response of the optoelectricaltransducer (400).
 16. The method as claimed in claim 15, characterizedin that the frequency response of the optoelectrical transducer (400) isdetermined by dividing the transducer measured value by a frequencyresponse value (Dm) of the electrooptical component (60).
 17. The methodas claimed in claim 1, characterized in that the pulse frequency (fp) ofthe optical pulses is generated by means of a first high-frequencysource, in particular a pulse generator (10), and the measurement signal(Smeas) is generated by means of a second high-frequency source, inparticular a sine-wave generator (70), the two high-frequency sources(10, 70) being coupled, in particular coupled in phase-locked fashion.18. The method as claimed in claim 1, characterized in that the phaseresponse of the electrooptical component (60) is additionally measured.19. The method as claimed in claim 18, characterized in that a phasesignal (PL1) is generated which specifies the phase angle (ΔΦ1) betweenthe drive signal (SA) of a pulsed laser (20) generating the opticalpulses and the electrical measurement signal, the phase angle betweenthe generated phase signal (PL1) and the phase angle of the detectedmixed product (M″) is measured for each of the measurement frequencies(fmeas) in each case with formation of a phase measured value (ΔΦ2). 20.The method as claimed in claim 18, characterized in that the phaseresponse of the optoelectrical transducer (400) is additionallymeasured.
 21. An arrangement having a pulsed laser (20), anelectrooptical component (60) and a measuring device (100) having anevaluation device (120), which is suitable for carrying out a method asclaimed in claim 1.